feat(11-01): add ReportBranding model and BrandingHtmlHelper with tests

- Add ReportBranding positional record bundling MspLogo and ClientLogo
- Add BrandingHtmlHelper static class generating flex branding header HTML
- Add BrandingHtmlHelperTests covering all 4 logo states (null, both null, single, both)
- Add InternalsVisibleTo for SharepointToolbox.Tests in project file

SharepointToolbox/Core/Models/ReportBranding.cs

@@ -0,0 +1,8 @@
+namespace SharepointToolbox.Core.Models;
+
+/// <summary>
+/// Bundles MSP and client logos for passing to export services.
+/// Export services receive this as a simple DTO — they don't know
+/// about IBrandingService or ProfileService.
+/// </summary>
+public record ReportBranding(LogoData? MspLogo, LogoData? ClientLogo);